I have a 10mmx8mm coil, with a free air inductance of about 2.6uH and a resistance of ~2ohms. If I connect to the LDC2114EVM board, with its 47p capacitor, I can get reasonable results detecting an aluminium can. The coils oscillates at 14MHz. But running the calculations, this setup would result in a Q of 118 and an Rp of 28k - well outside the datasheet spec.
If I take the exact same coil, and resonate it with 377pF, it oscillates at 6.6MHz , Q is 31 and Rp is 2k - BUT IT DOESN"T WORK AS A SENSOR! The data stream just shows a steady output near zero (+/- 1 count), even though I can see the oscillation frequency rise as I bring the aluminium can close to the coil.
Can anyone explain this behaviour?
Note: channel one on this chip doesn't seem to work at all (no oscillation) - could I have a damaged chip that is causing the coil issues?